In many fields of communication, as well as in radar applications, interfacing front-ends to related circuitry in active array antennas is required. The front-end of a pulsed radar or time division duplex communication system typically includes a transmit branch and a receive branch. The transmit branch generally consists of a driver amplifier, power amplifier and filtering, while the receive branch generally consist of a low noise amplifier and a signal filtering arrangement. A transmit/receive switch device is used to select branch depending on whether the front-end should receive or transmit signals.
The performance of such a radar or communication system is heavily dependent on transmitted output power, power added efficiency in transmit mode, and noise figure in receiving mode. The transmit/receive switch of the front-end is thus a critical component since it must be designed to both withstand high output power while providing low-loss in order to minimize noise figure and maximize power added efficiency.
Active array antennas, either in a radio communications or a radar system, may require that a plurality of front-ends are placed in a tight grid; typically with half the operating wavelength apart. For active array antennas that require very high output power, it is not feasible to integrate a large number of front-ends in single-chip. Instead, the front-ends need to be placed on a carrier board and connected to a beam-former using transmission lines on the carrier board. For a transceiver front-end, either two separate receive and transmit lines or a bi-directional transmit/receive line can be used. The latter would require less routing area at the cost of increased loss in the extra transmit/receive switch required in the beam-former and front-end.
U.S. Pat. No. 6,009,314 discloses transmit/receive switch interfacing circuits having differential outputs/inputs to a single-ended antenna. For an alternative arrangement, a differential low-level interface, e.g. from a beam-former, is interfaced to single-ended circuits, e.g. power amplifiers and low noise amplifiers.
For highly integrated circuits made using silicon technologies, e.g. CMOS (Complementary Metal Oxide Semiconductor) and/or SiGe (Silicon Germanium), it is common practice to use differential interfaces rather than single-ended interfaces. Differential interfaces increase the output power, lowers parasitic inductance and increases isolation. The main drawback is that the transmit/receive switch in the front-end must be duplicated which increases front-end size and incurs extra loss.
There is thus a desire to provide transmit/receive switches for a differential bi-directional interface in the front-end circuit with decreased loss. Such a switch normally comprises a balun (balanced-unbalanced) arrangement, for example as described in “New Classes of Miniaturized Planar Marchand Baluns” by Wael M. Fathelbab, and Michael B. Steer, IEEE Transactions on Microwave Theory and Techniques, vol, 53, No. 4, April 2005. There is a desire to provide an enhanced balun arrangement that enables transmit/receive switches for a differential bi-directional interface in the front-end circuit to be designed with lower loss.